Oxidation of aromatic compounds containing oxygenated side chains



Patented Mar. 4, 1952 OXIDATION OF AROMATIC COMPOUNDS CONTAININGOXYGENATED SIDE CHAINS William G.-Toland, (in, Richmond, Calif.,assignor to California Research Corporation, San Francisco, Calif., acorporation of Delaware No Drawing. Application June 17, 1950, SerialNo. 168,850

11 Claims. (01. 260-524) This invention relates to the oxidation oforganic compounds, more particularly to the oxidation of partiallyoxidized alkyl aromatic hydrocarbons to produce aromatic carboxylicacids. The oxidation of partially oxidized derivatives of alkyl aromatichydrocarbons to produce aromatic carboxylic acids having the carboxylcarbon atom or atoms directly attached to a nuclear carbon atom presentsthe organic chemist with a delicate problem of control if high yields ofthe desired acids are to be obtained. If the conditions under which theoxidation is conducted are too severe, considerable losses of thecharging stock are sustained by reason of ring rupture of the aromaticnuclei. If the conditions under which the oxidation is conducted are notsufiiciently severe, large proportions of the charging stock areconverted to oxidation products which are intermediate between thecharging stock and the desired carboxylic acids.

It is an object of this invention to provide a method by which partiallyoxidized alkyl aromatic hydrocarbons characterized by an oxidation levelintermediate between the alkyl aromatic hydrocarbons and aromaticcarboxylic acids may be converted to aromatic carboxylic acids havingthe carboxyl atom or atoms directly attached to nuclear carbon atoms inapproximately theoretical yields.

It has now been found that partially oxidized derivatives of alkylaromatic hydrocarbons, .especially those having at least one hydrogenatom attached to the alpha carbon atom of an alkyl group. may beoxidized to produce aromatic carboxylic acids and aromatic carboxylicacid salts having the carboxyl carbon atom or atoms directly attached toa nuclear carbon atom by contacting the partially oxidized alkylaromatic hydrocarbon with sulfur and an aqueous solution of a basicmaterial selected from the group consisting of alkali metal hydroxidesand alkaline earth metal hydroxides at a temperature above about 550 F.The sulfur may be introduced into the reaction mixture as such or in theform of a water-soluble polysulfide or certain other inorganic sulfurcompounds which, in the presence of the reactants and under the reactionconditions, yield elemental sulfur. Such other sulfur compounds are moreparticularly described hereinafter.

The invention may be understood upon conside eration of the followingexamples of the oxidation of toluic acids to phthalic acids which areprovided to illustrate the character of the reaction which produce thedesired aromatic car- 2 p boxylic acids and their derivatives, but whichare not intended in and-of themselves to mark out the limits of theinvention.

I Example 1 'prep'aredby air oxidation of mixed xylenes and separatedfrom the reaction product mixture by distillation. The mol ratio oftoluic acid to sulfur to sodium hydroxide charged to the bomb was 1:421.After charging, the bomb was sealed and heated to a temperature of 600F. The bomb was then rocked for a period of 1 hours. During the heatingand rocking the pressure in the bomb rose to a maximum value of 2200 p.s; i. g. The bomb was'then cooled, depressured, opened, and its contentswere removed. 'The reaction product mixture consisted of an aqueousslurry of sodium phthalate and phthalic acid, hydrogen sulfide, andsulfur in the form of sodium polysulfidej The slurry was refluxed toboil out hydrogen sulfide and to precipitate excess sulfur. Sufficientaqueous caustic was added to bring the pH of the mixture to a value ofseven. The resulting solution was filtered and hydrochloric acid wasadded to the filtrate in amount sufficient to precipitate substantiallyall of the phthalic acids from the filtrate. During the reaction 87% ofthe toluic acid charged was converted to phthalic acids. The yield ofphthalic acids obtained was equivalent to 115.2 weight per cent of thetoluic acids charged.

Example 2 In this example the reaction was conducted in a continuousmanner in a reaction system consisting of a reactor section of 40 ft. ofstainless steel tubing wound as a coil and immersed in a molten metalbath. The inside diameter of the tubing was 1"; inch. The partiallyoxidized alkyl aromatic hydrocarbon material constituting the feed wasthe sodium hydroxide extract of the reaction product obtained during airoxidation of mixed xylenes containing by volume of metaand p-araxylenes.This feed mixture together with sulfur in the form of sodium polylllfidewas pumped into the coil. From the coil the reaction product entered asurge chamber consisting of a 2.4 liter stainless steel bomb held at anintermediate temperature to avoid further reaction and still maintainthe reaction products in solution. A back pressure of nitrogen wasmaintained on this surge vessel to control operating pressures. Gaseswere bled from the surge vessel through a pressure regulator as requiredto maintain the pressure within tolerable limits. Upon completion of therun the products in the surge vessel were bled under pressure to a flashchamber from which most of the hydrogen sulfide formed during thereaction escaped overhead and in which the liquid products collected.The liquid products were then drained from the flash chamber andphthalic acids were recovered in the manner described in Example 1. Theratio of toluic acids to sulfur to sodium sulfide introduced into thecoil was 126.2133. The reaction time, as determined by dividing thereactor volume by feed rate of the reactants, was /2 hour. The reactiontemperature was 605 F. and the maximum press re observed during thereaction period was 2150 p. s.,i. g. Phthalic acids were recovered fromthe reaction product mixture in amount equal to 83 by weight of thetoluic acids charged.

Example 3 120 gm. of acetophenone (1.0 mol), 80 gm. of sulfur. and asolution of 40 gm. of NaOH in 200 cc. of water were charged to a 2.5liter bomb. The bomb was sealed and heated to 480 F., at which temerature shaking was begun. The pressure in the bomb rose from 300 To. s.i. to 1300 p. s. i. as the temperature rose from 480 F. to 585 F. over aperiod of 50 minutes. The bomb was cooled,. opened, and the productswere removed. There was a large amount of heavy black oil that did notdissolve in the aoueous alkaline solution. It was separated, .dried, andfound to weigh 108 gm. The acueous solution was acidified to pH 5.0 andthe resulting precipitate of yellow sulfur filtered oif. Filtrate wasstrongly acidified with HCl while hot and a large voluminous precipitatewas obtained. The mixture was cooled, filtered, and the filter cakewashed twice with water and dried. 19 gm. of white needle-like crystalshaving a neutral equivalent of 110 were recovered. After sublimationthis material had an M. P. of 120 C. and an N. E.=122. (N. E. benzoicacid l22.)

The oxidation of the toluic acids and acetophenone as conducted in theforegoing examples illustrate the manner in which partial oxidationproducts derived from alkyl aromatic hydrocarbons may be oxidized toaromatic carboxylic acids having the carboxyl carbon atom directlyattached to a nuclear carbon atom. Other partial oxidation productsderived from alkyl aromatic hydrocarbons such as aromatic alcohols,aromatic ketones, aromatic aldehydes, alkyl aromatic alcohols, alkylaromatic ketones, alkyl aromatic aldehydes, and alkyl aromaticcarboxylic acids may be oxidized to aromatic carboxylic acids by themethod illustrated in the examples. Where the alkyl aromatic alcohol,aldehyde or ketone is a product of the partial oxidation of an alkylaromatic hydrocarbon having at least one hydrogen atom attached to twoor more alpha carbon atoms of alkyl groups, aromatic polycarboxylicacids are produced during the reaction.

When luic acid is oxidized urs 139 process of the invention, thereaction appears to be as follows:

COOH

as zmo OH: OOOH Where a higher alkyl benzoic acid is oxidized pursuantto the invention, the reaction product mixture contains carbon dioxideproduced by the oxidation of the alkyl carbon atoms other than the alphacarbon atom in the manner illustrated by the following equation for theoxida. tion of ethylbenzoic acid:

zHa COOH as follows:

Where benzyl alcohol is oxidized by the process of the invention, thereaction appears to proceed as follows:

CH1OH O O OH While the above equations indicate only the; presence ofsulfur and water and their consumption during the oxidation of thecharging stocks, it has been found necessary to have substantial amountsof a basic material such as an alkali metal hydroxide or an alkalineearth metal hy- 0 droxide present in the reaction mixture. The role ofthe basic material in the reaction has not been definitely established,but it appears to function as a solvent for both the charging stock andthe sulfur, and intermediate reactions may occur in which the basicmaterial participates without being consumed. When it is attempted tooxi-.- dize the partially oxidized alkyl aromatic hydrocarbonderivatives described above in the absence of a basic material, thereaction proceeds as if water were not present, producing stilbene andthiophene derivatives. Toluic acid, water and sodium hydroxide do notreact at all under the conditions of the above examples in the absenceof sulfur.

The materials which may be oxidized to aromatic carboxylic acids havingthe carboxyl carbon atom or atoms directly attached to nuclear carbonatoms include, in addition to the materials specifically exemplifiedabove, various intermediately oxidized materials produced by partialoxidation or other alkyl and polyalkyl benzenes and alkyl and polyakylnaphthalenes without limitation as to the number or length of the alkylside-chains. The alkyl aromatic hydrocarbons themselves may (:5 beconverted to aromatic carboxylic acids by theof the invention, thereaction appears to proceed process of the invention, but the yields arevery much lower than those obtained when a partially oxidized chargingstock is treated. It is believed that the reaction proceeds more readilywith the partially oxidized derivatives of alkyl aromatic hydrocarbonsbecause they are substantially more soluble in water and in aqueousalkaline solutions than are the hydrocarbons themselves. Indications arethat higher yields of aromatic carboxylic acids may be obtained from thehydrocarbons if the reaction mixture is subjected to extremely'violentagitation during the reaction period.

It has been observed that the tertiary butyl group is more resistant tothe reaction of this invention than other alkyl configurations and thatpartial oxidation products derived from alkyl aromatic hydrocarbons inwhich the alpha carbon atom of an alkyl side-chain is attached to atleast one hydrogen atom are more rapidly and completely responsive tothe reaction of the invention than those having a quaternary alphacarbon atom in the alkyl side-chain.

Sulfur can be introduced into the reaction mixture as such, or in theform of a water-soluble polysulfide, as shown in the above examples, or

in the form of a sulfur compound which is convertible to sulfur underthe conditions of the reaction. For example, hydrogen sulfide and sulfurdioxide may be introduced into the reaction mixture in lieu of sulfurand an identical reaction is obtaind. Sulfur dioxide or a watersolublesulfite may be employed in the reaction mixture as the primary source ofsulfur for the oxidation. small amount of elemental sulfur, not usuallyexceeding about 6% of the amount which would be required if elementalsulfur were to be used as.

When these materials are used, a

the sole oxidizing agent, may be introduced into The sulfur may also beintroduced into the reaction mixture in the form of thionic acids andtheir salts, or sodium thiosulfate may be employed as the source ofsulfur. These materials decompose under the conditions of the reactionto produce sulfate, sulfite, and sulfur, and'the sulfite and sulfur areconsumed in the oxidation reaction. In general, any inorganic sulfurcompound containing at least one sulfur atom which is at a valence levelbelow plus 6 and above minus 2 may be introduced into the reactionmixture as the primary source of sulfur and where such materials areused a small amount of elemental sulfur is desirably but not necessarilyintroduced into the reaction mixture to act as a catalyst or initiator.

While sulfur acts as the oxidizing agent in-the reaction, the oxygennecessary to form the carboxyl groups of the acidic reaction product issupplied by water; consequently, water must be present in the reactionmixture. It is desirably present in amounts greatly exceeding the amounttheoretically required to supply the necessary oxygen.

The proportions of the reactants which are desirably present in thereaction mixture, except for the basic material, may be determined froma balanced equation for the oxidation reaction in which sulfur acts asthe oxidizing agent and water supplies the oxygen to the carboxyl groupswhich are formed. Where complete oxidation of kali metal hydroxides,especially potassium hydroxide; and the alkaline earth metal hydroxides,

especially magnesium hydroxide, calcium hydroxide and barium hydroxide;also, their salts with weak inorganic acids as carbonates, bicarbonates,sulfides, and sulfites. The amount of the basic material which ispresent in the reaction mixture may be varied over a considerable range.At least sufficient basic material to react with all of the carboxylgroups formed in the course of the reaction to form the metal salts ofthe acids should be present. The basic material is preferably presentsomewhat in excess of this amount; larger amounts of basic material, forexample, five to six times the amount necessary to neutralize thecarboxylic acids formed, have been employed with good effect. The basicmaterial may be entirely in solution in water or it may be employed inthe form of a dispersion or slurry. The moderately soluble alkline earthmetal hydroxides are suitably introduced into the reactor in the form ofan aqueous slurry.

The reaction should be conducted at temperatures above about 550 F. Atlower temperatures the rate of reaction is slow, and at 500 F. noreaction appears to occur. The maximum temperature of reaction should bebelow the critical temperature of water in order that a liquid aqueousphase may be present in the reaction'mixture. Preferably, the reactionis conducted at temperatures in the range about 500 F. to 675 F., 580 F.to 620 F. being optimum.

Pressure is a dependent variable in the reaction; it is usually in therange about 1000 to '3000 p. s. i. g. which insures the presence of aliquid aqueous phase. The pressure may be controlled during the reactionby bleeding'off a portion of the gas formed during the reaction; thishas the additional effect of reducing the partial pressure of hydrogensulfide in the reactor.

The purification of the reaction products was generally described inExample 1 above. Variable percentages of sulfur are sometimesencountered in the reaction products. The amount of sulfur present doesnot ordinarily exceed 3% by weight of the product and appears to be afunction of the amount of excess sulfur used for the oxidation and thecare'taken during the neutralization step. This sulfur is produced inpart by acid decomposition of thiosulfate formed when sulfur is digestedin alkaline solutions. Ordinarily, the thiosulfate is consumed duringthe oxidation unless large excesses of sulfur are employed. If it is notconsumed it decomposes upon acidification oi the products at elevatedtemperatures and at pH values below 3 to 4 to give free sulfur andsulfur dioxide. These factors may be controlled to minimize itsformation and decomposition so that sulfur-free products are obtained.In the event that the product is contaminated with elemental sulfur,this impurity may be removed by redissolving the acidsin caustic,filtering to remove the free sulfur, and reprecipitating the acids bythe addition of a strong acid.

.auazeoe Hydrogen sulfide produced during the reaction may berecoveredfor re-use. It may be absorbed in aqueous sodiumhydroxide-which, under'c'ertain conditions, may be air blown to producesodium polysulfide which is introduced into the reaction mixture.

I claim:-

1. A process for producing aromatic carboxylic acids and salts ofaromatic carboxylic acids having the carboxyl carbon atom directlyattached to a nuclear carbon atom which comprises oxidizing anoxygen-containing product of the partial oxidation of an alkyl aromatichydrocarbon by contacting said product with an aqueous mixture of abasic material selected from the group consisting of alkali metalhydroxides and their salts with weak inorganic acids, alkaline earthmetal hydroxides and their salts with weak inorganic acids, and aninorganic sulfurous material containing sulfur atoms at a valence levelbelow plus six and above minus 2 at a temperature above about 550 F.

2. A process forproducing aromatic carboxylic acids and salts ofaromatic carboxylic acids having the carboxyl carbon atom directlyattached to a nuclear carbon atom which comprises oxidizing anoxygen-containing product of the partial oxidationof an alkyl aromatichydrocarbon having at least one hydrogen atom attached to the alphacarbon of an alkyl group by contacting said product with an' aqueousmixture of a basic material selected from the group consisting of alkalimetal hydroxides and alkaline earth metal hydroxides and an inorganicsulfurous material containing sulfur atoms at a valence level below plussix and above minus 2 at a temperature above about 550 F.

3. The method as defined in;claim 1, wherein the product of partialoxidation is an alkyl benzene carboxylic acid.

4. The method as defined in claim 1, wherein the product of partialoxidation of the alkyl aromatic hydrocarbon charged is an aromaticalcohol.

5. The method as defined in claim 1, wherein the product of partialoxidation of the alkyl aromatic hydrocarbon charged is an aromaticaldehyde.

8 6. The method as defined in claim 1, wherein the product of partialoxidation ofthe'alkyl aromatic hydrocarbon charged is an aromaticketone.

7. The method as defined in claim 1, wherein the inorganic sulfurousmaterial comprises hydrogen sulfide and sulfur dioxide.

8. A process for producing aromatic polycar boxylic acids having thecarboxyl carbon atom 01' atoms directly attached to a nuclear carbonatom which comprises contacting an alkyl aromatic carboxylic acid havingat least one hydrogen attached to the alpha carbon atom of an alkylgroup with an aqueous solution of an alkali metal hydroxide and amaterial selected from the group consisting of elemental sulfur,water-soluble polysulfides, mixtures of water-soluble sul- .fites andelemental sulfur, and mixtures of hy- '11. The method asdefined in claim8, wherein I the alkyl aromatic carboxylic acid is an alkyl phthalicacid.

WILLIAMG. TOLAND. JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date v 2,470,876 Spindt et a1. May 24,1949 FOREIGN PATENTS Number Country Date 364,442 I Germany Nov. 25, 1922OTHER REFERENCES Friedmann, Chem. Abstracts, vol. 36, col. 885 (1942).

1. A PROCESS FOR PRODUCING AROMATIC CARBOXYLIC ACIDS AND SALTS OFAROMATIC CARBOXYLIC ACIDS HAVING THE CARBOXYL CARBON ATOM DIRECTLYATTACHED TO A NUCLEAR CARBON ATOM WHICH COMPRISES OXIDIZING ANOXYGEN-CONTAINING PRODUCT OF THE PARTIAL OXIDATION OF AN ALKYL AROMATICHYDROCARBON BY CONTACTING SAID PRODUCT WITH AN AQUEOUS MIXTURE OF ABASIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF ALKALI METALHYDROXIDES AND THEIR SALTS WITH WEAK INORGANIC ACIDS, ALKALINE EARTHMETAL HYDROXIDES AND THEIR SALTS WITH WEAK INORGANIC ACIDS, AND ANINORGANIC SULFUROUS MATERIAL CONTAINING SULFUR ATOMS AT A VALENCE LEVELBELOW PLUS SIX AND ABOVE MINUS 2 AT A TEMPERATURE ABOVE ABOUT 550* F.